EFFECTS OF ELECTRICAL STIMULATION OF RAT MIDBRAIN ON 5‐HYDROXYTRYPTAMINE SYNTHESIS AS DETERMINED BY A SENSITIVE RADIOISOTOPE METHOD

— 5-Hydroxytryptamine (5-HT) synthesis has been determined in the rat brain by measuring the 5-[3H]HT formed from [3H]tryptophan in the presence of monoamine oxidase inhibitor. Electrical stimulation in the region of the midbrain raphe nucleus increased formation of 5-[3H]HT by over 100 per cent, although the level of endogenous 5-HT and the concentration and specific activity of tryptophan were unchanged; the results are interpreted in terms of a two-compartment model. The optimum stimulation parameters were determined. Three days after a single dose of the tryptophan hydroxylase inhibitor p-chlorophenylalanine, stimulation increased 5-HT synthesis by the same percentage as in untreated animals. It was also found that after the end of an hour's stimulation, synthesis returned to control values in under an hour. These results suggest that the rise in synthesis of the amine on stimulation is not due to induction of tryptophan hydroxylase, but more likely to an increase in the activity of existing enzyme.